This invention relates generally to semiconductor chip device assembly, and in particular to flip chip device assembly. More specifically, the invention relates to a multi-piece integrated heat spreader/stiffener assembly and method of its assembly in a semiconductor package which reduces stress and warpage of a semiconductor die during attachment of the die to a substrate.
In semiconductor device assembly, a semiconductor chip (also referred to as an integrated circuit (IC) chip or "die") may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g. wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is "flipped" onto its active circuit surface so that the solder balls form electrical connections directly between the chip and conductive traces on a packaging substrate. Semiconductor chips of this type are commonly called "flip chips".
FIGS. 1A-F illustrate stages in a conventional method for packaging a semiconductor flip chip, in which a semiconductor die and a packaging substrate are electrically connected and mechanically bonded. FIG. 1A shows a cross-sectional, side view of an unbonded flip chip with the chip 100 having an active circuit surface 102 on which are arranged solder balls 104. The solder may be composed of a low melting point eutectic material or a high lead material, for example. It should be noted that this figure and the figures that follow are intended to be representative and, for example, do not show the solder balls 104 in proportion to the semiconductor die 100. In current designs, the die may have dimensions on the order of 0.5.times.0.5 inch (1 inch=2.54 cm) whereas the unbonded solder balls may have a diameter on the order of 4 to 5 mils (1 mil=10.sup.-3 inch=0.0254 mm).
Prior to bonding the die 100 to a substrate, solder flux is applied to either the active surface 102 of the die 100 or the packaging substrate surface. The flux serves primarily to aid the flow of the solder, such that the solder balls 104 make good contact with traces on the packaging substrate. It may be applied in any of a variety of methods, including brushing or spraying, or dipping the die 100 into a thin film, thereby coating the solder balls 104 with flux. The flux generally has an acidic component, which removes oxide barriers from the solder surfaces, and an adhesive quality, which helps to prevent the die from moving on the packaging substrate surface during the assembly process.
As shown in FIG. 1B, after the flux is applied, the die 100 is aligned with and placed onto a placement site on the packaging substrate 106 such that the die's solder balls 104 are aligned with electrical traces (not shown) on the substrate 106. The substrate is typically composed of a laminate or organic material, such as fiber glass, PTFE (such as Teflon.TM., available form Gore, Eau Claire, Wis.) BT resin, epoxy laminates or ceramic-plastic composites. Heat (to a temperature of about 220.degree. C, for example) is applied to one or more of the die 100 and the packaging substrate 106, causing the solder balls 104 to reflow and form electrical connections between the die 100 and the packaging substrate 106. Then, the remaining flux residue is substantially removed in a cleaning step, for instance by washing with an appropriate solvent.
At this point, the mechanical bonding procedure can begin. An underfill material, typically a thermo-set epoxy 108, such as is available from Hysol Corporation of Industry, California (product numbers 4511 and 4527), Ablestik Laboratories of Rancho Domingo, Calif. and Johnson Matthey Electronics of San Diego, Calif., is dispensed into the remaining space (or "gap") 107 between the die 100 and the substrate 106. In a typical procedure, a bead of thermo-set epoxy, is applied along one edge of the die where it is drawn under the die by capillary action until it completely fills the gap between the die and the packaging substrate. Slight heating of the packaging substrate after dispensing of the underfill epoxy assists the flow. In some cases, the underfill epoxy flow is further assisted by vacuum, or, alternatively, by injection of the epoxy into the gap.
After the epoxy 108 has bled through the gap 107, a separate bead of epoxy (not shown) may also be dispensed and bonded around the perimeter of the die 100. Thereafter, the epoxy (both the underfill and perimeter bonding epoxy, if any) are cured by heating the substrate and die to an appropriate curing temperature, for example, about 130 to 165.degree. C. In this manner the process produces a mechanically, as well as electrically, bonded semiconductor chip assembly, with the underfill material 108 allowing a redistribution of the stress at the connection between the die 100 and the substrate 106 from the solder 104 joints only to the entire sub-die area. FIG. 1C shows the semiconductor die 100 interconnected to the packaging substrate 106 electrically by solder 104 joints and mechanically by a cured layer of epoxy 108.
Semiconductor packages are typically subject to temperature cycling during normal operation. In order to improve the thermal performance and reliability of the packages, a stiffener 110 may be placed around the die 100 on the substrate 106 where it is bonded with a heat curable adhesive (not shown), as shown in FIG. 1D. The stiffener 110 is typically a flat piece of high modulus (about 9.times.10.sup.6 to 30.times.10.sup.6 psi) metal about 10 to 40 mils thick, having substantially the same dimensions as the package substrate 106 with a window 111 in its center to clear the die 100. Typically, the stiffener 110 is composed of nickel-plated copper which has a coefficient of thermal expansion similar to that of typical substrate 106 materials. The stiffener 110 may be bonded to the substrate 106 prior to the placement, bonding and underfilling/curing steps previously described, or it may be bonded after the placement and bonding of the die 100, but prior to the underfilling/curing step. The stiffener is typically bonded and cured in a separate step following curing of the underfill material 108. The adhesive may also be cured concurrently with the curing of the underfill material in a single heating step.
The purpose of the stiffener 110 is to constrain the substrate 106 in order to prevent its warpage or other movement relative to the die 100 which may be caused by thermal cycling during operation of an electronic device in which the package is installed. Such movement may result from the different coefficients of thermal expansion (CTE) of the die 100 and substrate 106 materials, and may produce stress in the die or the package as a whole which can result in electrical and mechanical failures.
Next, as shown in FIG. 1E, a heat spreader 112, typically composed of a high thermal conductivity (about 2 to 4 W/cm.multidot.K) material, and having substantially the same dimensions as the package substrate 106, is attached over the stiffener 110 and the die 100 and bonded to each by a thermally conductive adhesive (not shown) which is also then heat cured. A conventional heat spreader is also typically a flat piece of nickel-plated copper about 20 to 40 mils thick. A conventional heat spreader 112 may not be applied until the underfill material 108 has been dispensed and cured because the heat spreader 112 prevents access to the die 100. Therefore, the heat spreader 112 is applied in a separate step following attachment of the die 100 and stiffener 110.
The purpose of the heat spreader is to disperse the heat generated during thermal cycling in order to reduce stress in the package due to different CTEs of the various elements of the package, including the die 100, substrate 106 and underfill 108. Since it covers and is attached to the die 100, the heat spreader 112 also plays a role is constraining the die 100 in place on the substrate 106, but only once the heat spreader 112 is attached in position following dispensation and curing of the underfill material 108.
In a final step of a conventional packaging process, shown in FIG. 1F, solder balls 114 are bonded to the underside 115 of the substrate 106. These solder balls 114 may be used to bond the chip package to a circuit board, such as a mother board, for use in an electronic application.
A problem with such flip chip package constructions is that during the heating and cooling involved in curing of the underfill material 108, the different CTEs of the substrate 106 and die 100 materials may stress these materials by causing them to warp or otherwise move relative to each other. In addition, the epoxy underfill 108 typically shrinks during the curing process, applying additional stress on the bonded die 100 and substrate 106 which may cause one or both to warp. As discussed above with reference to operational thermal cycling, such stress in the semiconductor package which may ultimately result in its electronic and/or mechanical failure, including cracking of the die 100.
Accordingly, what is needed are methods and apparatuses for improving the reliability of flip-chip packages by providing die constraint during the underfill curing process.